The overall goal of this proposal is to understand the molecular mechanisms underlying the effect of leptin to rapidly enhance insulin sensitivity independent of its effects on food intake and body weight. We will investigate the contribution of CNS-mediated and direct effects at the level of peripheral tissues. In Aim 1 we will determine whether insulin and leptin share common intracellular signal transduction pathways. With i.v. administration of leptin in rats, we will test the hypothesis that the insulin- sensitizing effects of leptin involve convergence of synergism between leptin-activated and insulin-activated signal transduction pathways. We will determine the time course for leptin activation of phosphorylation of the insulin receptor and IRSs and activation of PI3 kinase, MAPK kinase and Stat1 and Stat in insulin target tissues (skeletal muscle, BAT, WAT liver) in vivo. We will investigate potential additive effects of leptin and insulin. In Aim 2 we will test the hypothesis that early activation of signaling pathways by leptin results from direct effects the level of the target tissues but later effects involve CNS mediation as well. We will measure activation of signaling pathways in the hypothalamus after iv or ICV leptin over a time course. We will determine which effects are present after sympathectomy of skeletal muscle or BT. We will determine which signaling pathways are activated in vitro in adipocytes and muscle. In Aim 3 we will test the hypothesis that some of the biological actions of leptin are exerted directly at the level of the adipocyte. We will generate transgenic db/db mice expressing the leptin receptor selectively in fat using the aP2 promoter/enhancer. We will determine whether restoration of long form leptin receptors exclusively in adipocytes alters adipocyte physiology, glucose homeostasis, or insulin action. In im 4 we will test the hypothesis that specific hypothalamic nuclei are involved in the insulin-sensitizing effects of leptin. We will microinject leptin into the DMH, VMH and PVH and measure glucose disposal by euglycemic clamp and glucose uptake into specific muscles and adipose depots in vivo. Thus, we will map the regions of insulin sensitivity. These studies will yield important information about the direct and CNS-mediated mechanisms by which leptin enhances insulin sensitively and may lead to new avenues for treatment of obesity and diabetes.